1. Field of the Invention
The present invention relates to an information acquiring apparatus and an information acquiring method for acquiring mass-related information.
2. Description of the Related Art
For the use with the matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS), a solid or liquid sample is mixed with a substance that is referred to as matrix (e.g., sinapinic acid, glycerin and like others) and applied onto a metal-made sample holder. Then, the sample holder carrying the matrix containing the sample is introduced into a vacuum chamber. As a laser beam is irradiated as a primary probe onto the matrix, while a high voltage is being applied between the sample holder and an extraction electrode that is arranged above the sample holder, ingredients of the matrix absorb the laser energy to be gasified and emitted with molecules of the sample into the vacuum from the sample holder. In the course of this process, molecules of the sample are believed to be ionized as protons are transferred between molecules of the matrix and those of the sample to form secondary ions. The secondary ions that are formed in this way are then accelerated by the extraction electrode and the mass/charge ratio of the secondary ions can be determined by observing the time-of-flight of the secondary ions until they get to a detector.
On the other hand, the time-of-flight secondary ion mass spectrometry (TOF-SIMS) utilizes the same principle except that it differs from the MALDI-TOFMS in that the former does not use any matrix and its primary probe is different from that of the MALDI. With the TOF-SIMS, a sample holder on which a sample is arranged is introduced into vacuum and a primary ion beam is irradiated as a primary probe onto the sample, while a high voltage is being applied between the sample holder and an extraction electrode that is arranged above the sample holder. Then, in the course of this process, molecules of the sample are believed to be ionized as protons are transferred from the moisture or an organic ingredient contained in the sample to form secondary ions. The secondary ions that are formed in this way are then accelerated by the extraction electrode and the mass/charge ratio of the secondary ions can be determined by observing the time-of-flight of the secondary ions until they get to a detector. In the following, laser beams or primary ion beams as used in the above as a primary probe are referred to inclusively as “primary beam”.
Ionized molecules of the sample are usually detected as protonated molecules of the sample (or in the state that sample molecules are adhered with other charged particles; the state nevertheless being represented by protonated molecules in the following description) produced by way of the above described process. However, many of the emitted sample molecules end up without colliding with protons in their flights and hence without participating the observation. Meanwhile, with the electro-spray ionization mass spectrometry (ESI-MS), the sensitivity of detecting molecules of a sample is believed to be improved by causing protons generated to a large extent from a solvent such as water to adhere to molecules of the sample. Therefore, an improvement of detection sensitivity can be expected for MALDI-TOFMS and also for TOF-SIMS by promoting adhesion of protons to emitted molecules of a sample. Japanese Patent Application Laid-Open No. H08-145950 discloses a method of improving the sensitivity of detecting molecules of a sample by way of a process that includes (1) gasifying an aqueous solution containing molecules of the sample, (2) exciting water molecules by a corona discharge to generate protons, and (3) causing generated protons to adhere to molecules of the sample. Japanese Patent Application Laid-Open No. H09-320515 discloses a method of improving the sensitivity of detecting specific sample molecules by arranging an ion-capturing electrode above a sample substrate (the ion-capturing electrode being insulated from the sample substrate) and causing an ionic chemical reaction to take place in a generated electric field.
With a method of analytically observing the mass of sample molecules on a substrate such as MALDI or TOF-SIMS, many of the sample molecules emitted from the sample fly in a neutral state. Thereafter, protons adhere to molecules of the sample and electrically charged secondary ions are detected as described above. At this time, since both protons and sample molecules divergently fly away from the point of irradiation of the primary beam, there arises a problem that protons adhere to sample molecules only with a low probability. The above-cited Japanese Patent Application Laid-Open No. H08-145950 discloses a method of supplying protons by means of a corona discharge in order to solve the problem.
However, the proposed method is accompanied by a problem of requiring a mechanism for supplying protons to make the overall mass spectrometer to be used for the method a complex and bulky one. Additionally, since protons are directly fed into a vacuum chamber with the proposed method, the chamber becomes full of protons to give rise to a high background level for signal detection. A high background level is detrimental to the reliability of observation.
The method described in the above cited Japanese Patent Application Laid-Open No. H09-320515 of improving the sensitivity of detecting specific sample molecules by arranging an ion-capturing electrode above a sample substrate and causing an ionic chemical reaction to take place in a generated electric field, on the other hand, is accompanied by the problems as listed below. The problems are: that (1) substances that can be made to become involved in an ionic chemical reaction by the proposed technique are electrically charged ions and the technique cannot handle neutral sample molecules; and that (2) protons are made to adhere to neutral sample molecules only poorly efficiently in “an electric field that is perpendicular to a sample substrate” generated by the electrode provided above the substrate. The reason for the problem (2) will be described in detail below. Firstly, the trajectory of a primary beam is defined as primary beam axis and the point of intersection of the primary beam axis and the sample surface is defined as central point. Furthermore, the axis that passes the central point and is normal relative to the substrate is defined as central axis. With these definitions, protons and sample molecules divergently fly away from the central point as described above. Many of those protons and sample molecules fly divergently from the central point into a conical region having a vertex at the central point and a rotation axis that is disposed in axial symmetry with the primary beam axis with regard to the central axis. Then, “an electric field that is perpendicular to a sample substrate” as described in Japanese Patent Application Laid-Open No. H09-320515 can hardly draw the protons that have flown away effectively back toward the sample substrate. In other words, the probability with which protons are made to adhere to flying sample molecules can hardly be raised.